In a high pressure lamp, protective metal oxide layers on the inner wall of the quartz envelope



April 9, 1968 F. KOURY ETAL 3,377,498

IN A HIGH PRESSURE LAMP, PROTECTIVE METAL OXIDE LAYERS ON THE INNER WALL OF THE QUARTZ ENVELOPE Filed Jan. 5, 1966 FREDERIC KOURY JOHN E WAYMOUTH INVENTORS A TORNE United States Patent ABSTRACT OF THE DISCLOSURE In lamps containing mercury, halogen and light emitting metals to prevent gettering of the light emitting metals by the silicon dioxide which forms the arc tube, a transparent layer of the oxide of the light emitting metal is disposed upon the inner surface thereof. Additional layers of refractory oxide and calcium, aluminum and magnesium oxide can be disposed between the light emitting metal oxide coating and the arc tube Wall.

This invention relates to high pressure electric discharge devices and particularly those which emit light other than that which is generally produced by a device containing mercury alone as the fill. In particular, this invention relates to high pressure electric discharge devices which contain mercury, a halogen and a light emitting metal in a quartz arc tube.

In the prior art, high pressure electric discharge devices have been manufactured which contain a fill of mercury .alone. When a potential is imposed across the electrodes in the device, the mercury is ionized and emits its characteristics spectral lines, generally in the blue-green region. It has recently been discovered that these devices could be modified in their emission by the inclusion of metals other than mercury so that the light produced was the combined emission of mercury and the included metals. Blending of emissions in this manner can produce wide variation in colors and most importantly, a white light can be attained.

When using metals (other than mercury) in the fill, we have found that they tend to react with the quartz (SiO which withdraws them from the plasma. Hence, they are not available to emit their characteristic spectrum from the arc discharge. We have discovered that if a barrier layer of the oxide of the light emitting metal is placed upon the inside wall of the arc tube, that this gettering is reduced markedly. The oxide of the light emitting metal will form a barrier which establishes an equilibrium between the metal in the arc plasma and the arc tube wall.

For example, in the case of arc tubes coating fills of thorium or scandium as the light emitting metal, when an arc is formed in a non-coated tube, a film of the oxides of these metals will form on the wall. An appreciable fraction of the light-emitting metal originally added to the arc-tube may be consumed in forming this film, and is therefore no longer available to produce light. This film is believed to be caused by a chemical reaction between the halide of the light emitting metal and silica to form silicon tetraiodide. The tetraiodide has a high vapor pressure and eventually migrates into the arc plasma. When a cold lamp is started, the iodine goes into the gas phase and increases starting voltages. Furthermore, the silicon tetraiodide which is formed tends to deposit out upon the electrodes of the lamp and decompose into silicon as a .made of quartz. Sealed in the arc tube Patented Apr. a, 1968 molten blob. This deposit causes random crystal growths on the electrode which eventually is melted back to produce higher starting voltages.

Accordingly, the primary object of our inventionis to prevent the light emitting metals of high pressure electric discharge devices from being gettered thereby removing them from the arc stream.

A feature of our invention is providing a coating of the oxide of a light emitting metal upon the inner surface of an arc tube to establish a barrier layer of the oxide.

Further features of our invention include the coating of a quartz arc tube with a layer of zirconuim oxide and coating the zirconium oxide layer with a second layer of refractory metal oxide and further coating these layers with a layer of the oxide of the light emitting metal in the arc tube.

The many other objects, features and advantages of our invention will become manifest to those conversant with the art upon reading the following specification when taken in conjunction with the accompanying drawings wherein the preferred embodiments of our invention are shown and described by way of illustrative examples.

Of the drawing, the figure is an elevational view of an arc tube, partially broken away to expose the inside.

Referring now to the drawing, the arc tube is generally 12, at the opposite ends thereof, are main discharge electrodes 13 and 14 which are supported on lead-in wires 4 and 5 respectively. Each main electrode comprises a core portion which may be a prolongation of the lead-in wires 4 and 5 and may be prepared of a suitable metal such as for example molybdenum ortungsten. The prolongations of these lead-in wires 4 and 5, can be surrounded by molybdenum or tungsten wire helixes. If desired, a small sliver of a metal or low work function such as thorium can be disposed between the helix and the rod in each of the electrodes to reduce cathode drop. In some cases when its spectrum is not desired, or limitation of the spectrum is preferred, thoriated tungsten wire (containing about 2 to 4% thorium by weight) can be used or, of course, the thorium can be eliminated entirely.

An auxiliary starting probe or electrode 18, generally prepared of tantalum or tungsten is provided at the base end of the arc tube 12 adjacent the main electrode 14 and comprises an inwardly projecting end of another leadin wire.

Each of the current lead-in wires described have their ends Welded to intermediate foil sections of molybdenum which are hermetically sealed within the pinched sealed portions of the arc tube. The foil sections are very thin, for example approximately 0.0008 inch thick and go into tension without rupturing or sealing oif when the heated arc tube cools. Relatively short molybdenum wires 23, 24 and 35 are welded in the outer ends of the foil and serve to convey current to the various electrodes inside the arc tube 12. The pinched or flattened end portions of the arc tube 12 form a seal which can be of any desired width and can be made by flattening or compressing the ends of the arc tube 12 while they are heated.

The are tube 12 is provided with a filling of mercury which reaches pressures in the order of one half to several atmospheres during normal amp operation at temperatures of 450 to 700 C. Moreover, the fill contains a halogen, particularly iodine and except fluorine, added as the halogen per se or a halide of one of the metals in the arc tube. As the source of emission other than mercury,

we add a light emitting metal as will be described later. The amount of mercury added can be varied widely as we have indicated but we prefer to add approximately 2.5)(10' to 4.1 X 10- gram atoms of mercury per centimeter of arc length. However, substantial light emission can be obtained at filling pressures less than the abovestated limits. Furthermore, the light emission appears to be substantially independent of the amount of mercury metal added and hence the quantity of the latter can be reduced while lamp operation is still attained. As is conventional in the art, a quantity of rare gas such as helium, argon, neon, krypton or xenon at a pressure of about 25 millimeters of mercury is added to facilitate starting. In manufacturing arc tubes of lamps containing fills other than mercury alone, many metals can be included such as molybdenum, scandium, thorium, vanadium, yttrium, lanthanum and cerium.

If necessary, coated upon the arc tube is a transparent layer 1 of a mixture of zirconium oxide and yttrium oxide, the thickness of which is 400 to 1000 A. and serves as a base for subsequent coatings. The quantity of yttrium oxide which may be included with the zirconium is less than 15% by weight, depending upon the operating temperatures of the arc tube. Coated over the zirconium oxide base is a transparent layer 2 of refractory oxide, such as calcium,

magnesium or aluminum oxide, generally of 1200 to 3000 A. in thickness. They may be composed of one or more layers or coatings of any of the constituents. The layer 2 of refractory oxides provides a sheath which prevents the zirconium atoms from migrating into the arc stream.

Importantly, the zirconium atoms should be completely shielded from the arc plasma by the second coating to prevent their ionization.

Coated upon the inside of the arc tube 12 or the refractory layer 2, preferably, is a transparent layer 3 of the oxide of the light emitting metal generally about 1000 to 10,000 A. thick. If the light emitting metal in the arc tube is scandium, for example, the light emitting metal oxide layer 3 is scandium oxide and so forth for the other metals mentioned heretofore.

It is apparent that modifications and changes may be made within the spirit and scope of the invention but it is our intention however only to be limited by the spirit and scope of the appended claims.

As our invention we claim:

1. A high pressure electric discharge device comprising: an arc tube and electrodes disposed at either end thereof; a fill including halogen atoms, mercury atoms and atoms of at least one light emitting metal; transparent light emitting metal oxide means disposed on the inner surface of said are tube for inhibiting the gettering of said light emitting metal by the glass of said are tube.

2. The device according to claim 1 wherein there is a transparent layer of zirconium oxide beneath said means.

3. The device according to claim 2 wherein yttrium oxide is substituted for zirconium oxide in quantities up to 15% by weight.

4. The device according to claim 2 wherein a transparent layer of refractory oxide selected from the group consisting of calcium oxide, aluminum oxide and magnesium oxide is interposed between said zirconium oxide layer and said means.

5. The device according to claim 1 wherein said light emitting metal is atoms of at least one of the metals selected from the group consisting of molybdenum, scandium, thorium, vanadium, yttrium, lanthanum and cerium.

6. A high pressure electric discharge device comprising: an arc tube and electrodes'disposed at either end thereof; a fill including halogen atoms, mercury atoms and atoms of at least one light emitting metal; a transparent layer of the oxide of at least one of the light emitting metals of the fill disposed inside of said arc tube.

7. The device according to claim 6 wherein there is a transaprent layer of zirconium oxide beneath said layer of oxide.

8. The device according to claim 7 wherein yttrium oxide is substituted for zirconium oxide in quantities up to 15% by weight.

9. The device according to claimr7 wherein a transparent layer of refractory oxide selected from the group consisting of calcium oxide, aluminum oxide and magnesium oxide as interposed between said zirconium oxide layer and said layer of oxide.

10. The device according to claim 6 wherein said light emitting metal is atoms of at least one of the metals selected from the group consisting of molybdenum, scandium, thorium, vanadium, yttrium, lanthanum and cerium.

References Cited UNITED STATES PATENTS 2,177,728 10/1939 Krefft et al. 313-221 X 2,238,777 4/ 1941 Lemmers et al 313-221 2,568,459 9/1961 I Noel 313-221 FOREIGN PATENTS 947,311 1/ 1964 Great Britain.

JAMES W. LAWRENCE, Primary Examiner. C. R. CAMPBELL, Assistant Examiner. 

